This invention relates to surface acoustic wave devices, and in particular to an improved method and means for realizing surface acoustic wave filters that exhibit linear phase and flat in-band-exponentially decaying out-of-band frequency response characteristics.
Surface acoustic wave technology has become attractive for a wide variety of signal processing applications due to factors such as compactness, light weight, reliablitiy, and advantages of fabrication by well established planar techniques. Acoustic waves which travel along the surface of a piezoelectric solid can be tapped with interdigital transducers thus forming a transversal filter and providing a powerful tool for realizing some very useful filters not easily synthesized with other classical analog filter techniques.
Many filter response functions which are very difficult to realize with lumped element networks can be more easily achieved with surface acoustic wave devices. Filters for pulse applications which simultaneously achieve a flat magnitude and linear phase response are one example.
Frequently the requirements of these and other filter applications call for both excellent low distortion time domain properties and good frequency selectivity. In order to achieve this, there are required filters and filter design techniques that allow optimization of the trade off between achieving a low distortion time domain response while maintaining good frequency selectivity. Such optimization is particularly necessary when filtering pulses which have frquency spectra of the order of the filter bandwidth.
The present invention therefore is directed toward providing surface acoustic wave filters and design and construction techniques that meet such requirements and that permit virtually any trade off between time and frequency performance.